1. Field of the Invention
This invention relates to a method, device, and video signal recording medium which are made capable of anti-duplication control, for example, in the case that a video signal recorded on a recording medium is played back and transmitted together with an information for preventing duplication, and the recording of the transmitted and received video signal on another recording medium is inhibited or restricted, by way of a method involving a video signal on which the additional information is superimposed is output, the superimposed additional information is extracted from the received signal, and the extracted additional information is utilized to prevent duplication.
2. Description of the Related Art
VTR (Video Tape recording devices) has been popularized in daily life, and many kinds of software which can be played back on a VTR are supplied abundantly. Digital VTR or DVD (Digital Video Disks) playback devices have been available practically now, and provide images and sound of exceptionally high quality.
On the other hand, there is, however, a problem in that this great abundance of software can be copied without restriction, and several methods have already been proposed to inhibit duplication.
For example, fora VTR which outputs an analog video signal, one method to prevent copying uses a difference in the AGC (Automatic Gain Control) system, or in the APC (Automatic Phase Control) system, of the VTR recording device and of a monitor receiver for displaying the image.
When the method which utilizes the difference in AGC system, in which a VTR performs AGC using a pseudo sync signal inserted in the video signal and a monitor receiver employs a different AGC system not using the pseudo sync signal, is used, a very high level pseudo sync signal is inserted in the video signal supplied from the playback VTR and the video signal with insertion is output to the recording VTR as an AGC sync signal.
When the method which utilizes the difference in APC characteristics, in which a VTR performs APC using the phase of the color burst signal itself in the video signal and a monitor receiver employs an APC system different from that of the VTR, is used, the phase of the color burst of the video signal supplied from the playback VTR to the recording VTR is inverted partially.
As a result, the monitor receiver which receives the analog video signal from the playback VTR plays back the image correctly without being affected by the pseudo sync signal in AGC or the partial phase inversion of the color burst signal used for APC.
On the other hand, in a VTR, which is supplied with the analog video signal from the playback VTR into which pseudo sync signals have been inserted or which has been subjected to color burst signal phase inversion control as described herein above, for receiving such analog video signal and for recording the analog video signal on a recording medium, proper gain control or phase control based on the input signal cannot be performed, and so the video signal is not correctly recorded. Even if this signal is played back, therefore, normal picture and sound cannot be obtained.
In the case of a digitized video signal, for example, in a digital VTR, an anti-duplication signal or an anti-duplication control signal comprising, for example, a duplication ranking control code, is added as digital data to the video signal and recorded on the recording medium, so as to prevent or control duplication of the image.
In this case, the playback digital VTR reads the video signal, audio signal and anti-duplication control signal, and supplies them as digital or analog data to a recording digital VTR.
In the digital VTR used as a recording device, the anti-duplication control signal is extracted from the supplied playback signal, and recording of the playback signal is then controlled based on the anti-duplication control signal. For example, when the anti-duplication control signal comprises an anti-duplication signal, the recording VTR does not perform recording.
Alternatively, when the anti-duplication control signal comprises a duplication ranking control code, recording is controlled by this ranking control code. For example, when the duplication ranking code limits duplication to one copy, the digital VTR used for recording adds the anti-duplication code before recording the video signal and audio signal on the recording medium as digital data. It is thereafter impossible to duplicate the video signal from the copy.
Hence, in the case of a digital connection when the video signal, the audio signal, and the anti-duplication control signal used as digital signals are supplied to the digital VTR used as a recording device, anti-duplication control is performed on the recording side using the anti-duplication control signal by supplying this signal to the digital VTR as digital data.
However, in the case of an analog connection where the video signal and audio signal are supplied as analog signals to a digital VTR used as a recording device (the digital VTR performs A/D conversion), D/A conversion of a signal to be supplied to the recording device causes the loss of the anti-duplication control signal because the anti-duplication control signal is not superimposed on the analog information signal such as a video signal and audio signal. Hence, in the case of an analog connection, an anti-duplication control signal must be added to the D/A converted image or sound signal, and this addition causes deterioration of the video signal and audio signal.
It is, therefore, difficult to add an anti-duplication control signal and to extract it in the recorder for the purpose of anti-duplication control, without causing deterioration of the D/A converted video signal or audio signal.
Conventionally, therefore, in the case of an analog connection, duplication was prevented by an anti-duplication method using a difference in the AGC, or a difference in APC characteristics, between the VTR and the monitor receiver
However, in some cases, when anti-duplication is prevented using the above-mentioned difference in the AGC or a difference in APC characteristics between the VTR and the monitor receiver, depending on the type of AGC or APC characteristics in the recording side, the video signal may nevertheless be correctly recorded, in this case, it might happen that duplication cannot be prevented, or that the reproduced image on the monitor receiver is distorted. Further, it is troublesome to change the anti-duplication method depending on whether there is an analog connection or a digital connection.
To solve such problems, an anti-duplication method in which a spectrally spread anti-duplication control signal is superimposed on a video signal can be used for both digital connections and analog connections without deterioration of the image or sound which is played back.
According to this method, a PN (Pseudorandom Noise) sequence code (referred to hereinafter as PN code) used as a spread code is generated with a sufficiently short period and spectrally spread by multiplying it by the anti-duplication control signal. In this way, a narrow-bandwidth high-level anti-duplication control signal is converted to a wide-band low-level signal which does not affect the video signal or sound signal. This spectrally spread anti-duplication control signal is then superimposed on the analog video signal, and recorded on a recording medium. In this case, the signal to be recorded on a recording medium may be an analog signal or a digital signal.
In the case that the recording medium does not carry a recorded video signal on which a spectrally spread anti duplication control signal is superimposed but the recording medium carries a recorded video signal on which an anti duplication control signal is recorded together with the video signal in the different other system, in the playback device, the anti-duplication control signal is extracted from the playback signal, spectrally spread, and superimposed on the video signal to be output.
On the other hand, in the recording device side, phase control is performed on the input video signal so that a PN code having the same generation timing and phase as those of the PN code used for spectrally spreading the anti-duplication control signal, and inversion spectral spreading is performed for extracting the original anti-duplication control signal by multiplying the video signal on which the spectrally spread anti-duplication control signal is superimposed by the PN code. Hence, the duplication prevention control is performed based on the anti-duplication control signal extracted by inversion spectral spreading.
In this way, the anti-duplication control signal is spectrally spread and superimposed on the video signal as a wide-band low-level signal. It is therefore difficult for a person who wishes to illegally duplicate the video signal, to remove the anti-duplication control signal which is superimposed on it.
However, it is possible to detect and use the superimposed anti-duplication control signal by inversion spectral spreading. This anti-duplication control signal is therefore supplied to the recording device together with the video signal. On the recording side, the anti-duplication control signal is detected, and duplication is consistently controlled according to the detected anti-duplication control signal.
According to this method, as described herein above, the spectrally spread anti-duplication control signal is superimposed as a wide band, low level signal on the video signal, but it must be superimposed at a low level so that S/N ratio is higher than that of the video signal in order not to cause deterioration of the video signal.
To superimpose the spectrally spread anti-duplication control signal at a low level so that S/N ratio is higher than that of the video signal, and to be able to detect the anti-duplication control signal superimposed on the video signal in the recording device, the number of the PN codes (PN code length) required to spectrally spread a 1 bit anti-duplication control signal must be sufficiently large. The PN code length per bit of the anti-duplication control signal may also be expressed as a spread gain (spread factor) which is the ratio (T/TC) of a time width T per bit of the anti-duplication control signal to a time width TC of one part (one chip) of the PN code. As described hereinafter, this spread gain is obtained corresponding to the S/N ratio of the information signal on which the anti-duplication control signal is superimposed, in this case, corresponding to the S/N ratio of the video signal.
For example, when the S/N ratio of the video signal on which the anti-duplication control signal is superimposed is 50 dB, the anti-duplication control signal which is spectrally spread and superimposed on the video signal must be superimposed at a low level so that S/N ratio is higher than 50 dB, which is the S/N ratio of the video signal. Also, in order to detect the anti-duplication control signal superimposed on the video signal, its S/N ratio must be sufficient for the spectrally spread signal to be fully demodulated. If this S/N ratio is 10 dB, a spread gain of 60 dB (S/N ratio of 50 dB for video signal+S/N ratio of 10 dB necessary for detection) is required. In this case, the PN code length per bit of the anti-duplication control signal is 1 million code length.
In the case of a video signal on which a spectrally spread additional information is superimposed, spread gain can not be made small because of significant adverse effect of the superimposed additional information on the video signal and necessary S/N ratio required for extraction of the superimposed additional information.
To cope with this problem alternatively, if a large number of spread codes required for spectrally spreading an additional information per one bit is used, it takes a long time to perform inversion spectral spreading for extracting the spectrally spread additional information, and adequate control corresponding to the additional information superimposed on a video signal can not be performed.
For example, in the case of the anti-duplication control signal of inhibition of duplication, a video signal recording device which records a video signal until an anti-duplication control signal is detected completes recording of the supplied video signal before the anti-duplication control signal is detected.
Also in the case of the anti-duplication control signal of permission of duplication, a video signal recording device which does not record a video signal until an anti-duplication control signal is detected can not record the video signal supplied before the anti-duplication control signal is detected.
In view of the above-mentioned problem, it is an object of the present invention to provide a method, device, and recording medium which are capable of extracting rapidly and correctly a spectrally spread anti-duplication control signal superimposed on a video signal to eliminate the above-mentioned problem.